1. Field of the Invention
The present invention relates to a heat treatment apparatus for semiconductor wafers, used in various heat treatment steps of, e.g., thermally diffusing impurities and forming a thermal oxide film, in a process of manufacturing a semiconductor device and, more specifically, to control of cooling characteristics and cooling speed of semiconductor wafers placed in a heat treatment apparatus.
2. Description of the Related Art
FIG. 1 is a view schematically showing a constitution of a prior art heat treatment apparatus for semiconductor wafers.
As shown in FIG. 1, a wafer mounting jig 3 in which a number of semiconductor wafers 2 are placed, is incorporated in a reaction tube 1, and a heater 4 is provided outside the tube 1. Between the tube 1 and heater 4, a cooling gas path 6 for allowing cooling gas to flow in the direction of arrows 5 is interposed, and one end 6a thereof is connected to a radiator 9 by a tube 7 through a blower 8. The radiator 9 is connected to the other end 6b of the gas path 6 by a second tube 10.
According to the above-described constitution of the heat treatment apparatus, the semiconductor wafers 2 are placed in the wafer mounting jig 3, and the jig 3 is housed in the reaction tube 1. The heater 4 is then turned on to heat the tube 1, and the wafers 2 are thermally treated through various steps of manufacturing semiconductor devices from the wafers.
After the heat treatment, the heater 4 is turned off and the blower 8 is operated to drop the temperature of the wafers 2. The cooling gas is forced to circulate through the gas path 6, tube 7, blower 8, radiator 9 and tube 10 in the direction of arrows 5. The gas passing through the path 6 is cooled by the radiator 9, and the cooled gas is guided again to the path 6 via the tube 10. Thus, the inner side of heater 4 and the outer side of tube 1 are cooled, as are the semiconductor wafers 2 in the tube 1.
In the prior art heat treatment apparatus shown in FIG. 1, when the wafers are cooled quickly by air blow to improve the operating efficiency of the apparatus or control the junction of elements, the capacity of blower 8, that is, the driving capacity of cooling gas is increased to improve in cooling efficiency. In this situation, however, the cooling speed can be increased within a high-temperature zone between, e.g., 800.degree. C. and 1200.degree. C., whereas it cannot be increased within a low-temperature zone below, e.g., 800.degree. C.
The rate of occurrence of crystal defects, such as a slip of silicon crystal due to variations in temperature on the surface of each semiconductor wafer, is opposite to the above relationship between the cooling speed and temperature zone. Specifically, when the wafers are cooled at the same cooling speed, the composition of silicon crystal is easy to be changed because of a slip within the high-temperature zone between 800.degree. C. and 1200.degree. C., while a slip is difficult to occur within the low-temperature zone below 800.degree. C. Since, in the prior art heat treatment apparatus, the cooling speed cannot be controlled appropriately within the high-temperature zone, if the cooling speed is increased, there occurs a crystal defect, such as a change in composition due to a slip on a semiconductor wafer.
Since, furthermore, the prior art apparatus is so constructed that the cooling gas flows from the bottom of the gas path 6 toward the top thereof, the cooling speed of a semiconductor wafer 2 placed on the lower portion of the reaction tube 1, is higher than that of a semiconductor wafer 2 placed on the upper portion thereof. The variations in cooling speeds of semiconductor wafers 2 in the reaction tube 1 vary characteristics of semiconductor devices obtained from the wafers, even when the semiconductor wafers 2 are thermally treated in the same lot.